Exhaust size

Hey, just a off topic question. When your building horsepower or even in a stock motor, what kind of exhaust would be best for the application. Example if I had a LS 5.3 would it be in my best interest to put 4 inch exhaust straight pipe straight through? I know in a natural aspirated motor it would need some back pressure and what size exhaust pipe would I run if I had a turbo?

Engines dont need exahust back pressure whether is natural aspirated or forced induction, back pressure is always detrimental for the performance view of an engine. I am not sure if you mean only one 4 inch pipe for the eight cylinders or two 4 inch pipes, if you mean two pipes , is two big . One pipe of 3 inches per bank is more than enough for a street/strip 5.3 LS natural aspirated engine. For turbo application generally the turbine is the place where the most exhaust restriction occurs, so for simplicity because the subject is not so simple , the norm is put the exahust pipe after the turbine no more smaller than the turbine outlet, maybe one size bigger if is a complete exahust system full of bents.

As above you never want back pressure, manifold runner length and diameter can be tuned for pulse effects but you don't want static pressure, it is just lost power.

So to be more precise what would be the math equation behind the exhaust? For example if i went with 2 dual straght pipes it would have to be 1-1/2inchs because with one pipe it would be 3 inch. I don't know how picky I have to be to get the best flow without going to big.

The math behind the exhaust? There are books written on that topic -- you should start with:

scientific design of exhaust and intake systems

Since the exhaust interacts with the engine cam, many experts use simulation software to help design the exhaust system to complement the rest of the engine components. Something like this:

DynoSim 6

The exhaust system design is huge for a normally aspirated engine. How do I do it -- I pay a guy that uses all the above tools with my initial dyno results. The systems he's designed for me have added as much as 15% power for a traditional 4-1 equal length header.

I greatly appreciate everyone for the information. This will better me on everything I would like to do for my vehicle's.

Hi, I mean one pipe of 3 inches per bank , the LS engine have two banks of cylinders one per side , so two pipes of three inches is a ballpark estimate for a 5.3 LS n.a street/strip engine.

What sort of power level are you expecting? A good 2" system, with a proper cross-over/"X" pipe and good mufflers should be close to the best all round setup - for a STRONG engine, a max' of 2 1/2" should be more than enough.

Remember, the pipe cross section is the square of the diameter - ignoring wall thickness (because I'm lazy, and it just increases the difference, anyway) if you take 2" as the datum, a 2 1/4" is 25% larger, a 2 1/2" is 56% bigger, a 3" is 125% bigger, and a 4" is 300% bigger.

It may sound strange, but because of turbulent gas flow, scavenging, and other stuff, a too-big pipe can cost more peak power, and torque, than a slightly too small one. Some of the same principles as used for sizing ports applies. You are probably familiar with oversized collectors killing torque and, often, power - exactly the same thing!

With a N/A LS the exhaust selection will depend on the rest of your aims. We have found that a 2.5" system is usually the best all round compromise for power and sound however if you're planning to go with a wild cam then a 3" system offers more power at the expense of more noise. Particularly for a street car we tended to suggest the 2.5" system as a 3" creates quite bad drone in the cabin, particularly at cruise speeds on the open road, and that wears thin really quickly. Your neighbours may also object, particularly if you tend to leave the house early/arrive home late ;)